By Julie Curnow

What size air conditioner do I need?

The sizing of a ducted reverse cycle air conditioner is difficult to work out because it depends upon so many factors:

  • The construction of your home
  • The design and orientation of your home
  • Number and usage of air conditioning zones (Zones separate your home into different areas so the air conditioning can be turned on or off at each zone) and
  • Any other things that may make it easier or harder to heat or cool (treat) your home.

In Australia, the rule of thumb is to use a cooling output between 125W/m2 and 145W/m2 for residential use and 180W/m2 for commercial use (watts per square metre).

*Refrigeration mechanics are taught to use 145W/m2 as part of their apprenticeship.

When you would use 125W/m2 to size your air conditioner:

Good passive design Australian 2 storey home

Good passive design Australian 2 storey home

When you would use less than 125W/m2 to size your air conditioner:

When your home would be sized at 125W/m2 and you have several other factors reducing the load on your air conditioner:

  • Use fans to boost the cooling and heating effect of your air conditioning
  • Your windows have little or no direct spring or summer sunlight and low levels of diffused light
  • Window panes designed to reduce the heat loss or gain
  • Fully sealed double glazing
  • Home predominantly used for heating (as the heating output is higher than the cooling output).
  • Living areas facing south
  • Super efficient insulation or build materials
  • House fully sealed and free of draughts
  • Tile roof
  • One or two people living in the home
  • Not many lights or electrical appliances running during the day.

When you would use between 125W/m2 -145W/m2 to size your air conditioner

Typical heat loss in a Perth home from

Typical heat loss in a Perth home, summer and winter

  • Ceiling over 2.4 metres high
  • Walls and ceiling partially, poorly or not insulated
  • No underfloor insulation, if not on a concrete pad
  • Bi-fold or French doors, large and plentiful windows
  • When door or windows are prone to being left open
  • Living areas face north or west
  • Home has dark walls
  • Dark roof
  • Metal roof
  • Four or more people living in the home.
  • Lots of electrical appliances and lights running all day.

At Air and Water Residential, we tend to err on the size of caution when determining the cooling output (wattage per square metre) because:

  • most units sold nowadays are inverters and they work more efficiently and cost less to run at less than full capacity,
  • there is less wear and tear on the air conditioning system if it is not spending a lot of time running at maximum capacity and it should thus last longer
  • it only costs a few hundred dollars more to purchase a bigger unit, which ends up being negligible when you expect the unit to last 10 – 15 years
  • no one wants an undersized conditioner. People want the air conditioner to cool sufficiently even on the super hot summer days where it is over 40 degrees centigrade.

How to calculate the size of air conditioner required (the heat load)

  1. Determine the cooling output you need (W/m2) based on the above guide
  2. Determine the rooms that you want treating (heating and cooling)  – This would normally be all rooms except the laundry, bathrooms and hallway.
  3. Decide on the zones you require – This is the tricky bit because you need to have some understanding of air flow and duct layout. Most homes have 2 – 8 zones with 4 being the most common for the average sized home.
  4. Measure the area (in m2) of your rooms being treated.
  5. Total the area to be treated by each zone
  6. Work out which zones you would want operating at the same time
  7. Calculate the area for each zone permutation.
  8. Identify the maximum area calculated for your zone permutations.
  9. Calculate the air conditioning output you require (in kW) – Multiply your cooling output (from step 1) by the maximum zone area (from step 8), then divide by 1,000.
  10. Determine which size air conditioner (in kW) you require – Look for the smallest air conditioner (should be part of the model description and will be in the brochure) with a cooling output that covers your requirements.


Example heat load calculation based on a cooling output of 145W/m2

This simple heat load calculation is based on a small home with only four rooms to be treated with each room a separate zone as there are only two occupants. The occupants wanted to be able to run two zones simultaneously.
As you can see running zones 1 and 2 together require the biggest area to be treated, being 45m2.
With a cooling output of 145W/m2  the maximum cooling output required for this home, using the largest  treatment area of 45m2 = 6.453kW
(45m2 x 145W/m2 divide by 1,000)
The air conditioner required is 7kW.

Don’t worry if you are feeling a bit daunted with these calculations. Any reputable air conditioning dealer will perform these calculations as part of their free air conditioning quote.

So all you need to do is ask for a copy of the Heat load calculation and the cooling output used in the heat load calculation.

Be mindful of quotes that have scrimped on the cooling output as that few hundred dollars saving in purchasing a smaller ducted reverse cycle air conditioning system may be at the cost of your comfort for the next 10 – 15 years!

This article was written by Don Curnow owner of Air and Water Residential, Perth Western Australia. Don has 18 years experience in the air conditioning industry in Perth and Melbourne. Air & Water supply a free heat load with all ducted reverse cycle air conditioning quotes. Air & Water size, design and uses components to ensure that your air conditioning is correctly sized to meet your lifestyle, based on your home.


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